The present invention relates to a method of molding a polyurethane product which has variable physical properties, such as, different colors, hardness, etc. between the outer and inner portions. The polyurethane materials applicable in the present invention are those which are known to those skilled in reaction-injection molding (RIM) methods.
The conventional RIM technology for molding polyurethane parts includes applying a mold release agent to the surface of the mold cavity. If necessary, an insert is set in the cavity of the mold. A RIM polyurethane material is then injected into the cavity so that the respective components will react to cure; and the mold is opened and the molded part is removed.
Among these steps, the application of the mold release agent accounts for a large portion of the molding cycle and hence, it is an impediment to reducing the cycle time. Furthermore, the use of a single type of RIM polyurethane material makes it difficult to provide different properties in the surface and the interior of the molded part (providing different blow ratios is all that can be presently accomplished). It has been impossible to achieve free alterations in various characteristics such as a feeling of touch and endurance. Although polyurethane molding by RIM is sometimes done after pigment-containing polyurethane materials (colorants) can be applied to the surface of the mold cavity, such an application step is time- consuming.